U.S. patent number 4,687,558 [Application Number 06/859,956] was granted by the patent office on 1987-08-18 for high current density cell.
This patent grant is currently assigned to Olin Corporation. Invention is credited to David D. Justice, Kenneth E. Woodard, Jr..
United States Patent |
4,687,558 |
Justice , et al. |
* August 18, 1987 |
High current density cell
Abstract
A filter press membrane electrolytic cell having at least one
cathode and one anode sandwiched about a permselective ion exchange
membrane which employs a cathode having a first layer and a second
layer cooperative with the membrane such that the voltage
coefficient during operation at current densities greater than 4.0
kiloamperes per square meter is less than about 0.20 volts per
kiloampere per square meter.
Inventors: |
Justice; David D. (Cleveland,
TN), Woodard, Jr.; Kenneth E. (Cleveland, TN) |
Assignee: |
Olin Corporation (Cheshire,
CT)
|
[*] Notice: |
The portion of the term of this patent
subsequent to May 13, 2003 has been disclaimed. |
Family
ID: |
27090303 |
Appl.
No.: |
06/859,956 |
Filed: |
May 5, 1986 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
626963 |
Jul 2, 1984 |
4588483 |
|
|
|
Current U.S.
Class: |
205/516; 204/253;
205/524; 205/535; 205/531; 205/534; 204/252; 204/283 |
Current CPC
Class: |
C25B
1/46 (20130101); C25B 11/03 (20130101); C25B
9/73 (20210101) |
Current International
Class: |
C25B
9/18 (20060101); C25B 1/46 (20060101); C25B
9/20 (20060101); C25B 1/00 (20060101); C25B
11/03 (20060101); C25B 11/00 (20060101); C25B
001/34 () |
Field of
Search: |
;204/1R,59,98,128,252,253,257,258,282,283,29R,294 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Dr. Dieter Bergner (Hoechst Aktiengesellschaft) Current-Voltage
Curves and Coefficient of Resistance in Commercial Chloralkali
Electrolysis-Chemiker-Zeitung 198(5), 177-183 (1985), pp.
1-22..
|
Primary Examiner: Niebling; John F.
Assistant Examiner: Chapman; Terryence
Attorney, Agent or Firm: D'Alessandro; Ralph O'Day; Thomas
P.
Parent Case Text
This is a continuation-in-part application of Ser. No. 626,963,
filed July 2, 1984, now U.S. Pat. No. 4,588,483.
Claims
What is claimed is:
1. In a filter press membrane electrolytic cell having at least one
cathode and one anode sandwiched about a permselective ion exchange
membrane having a first side adjacent the cathode and a second side
adjacent the anode, the improvement comprising in combination:
a. a dual cathode having a first layer and a second layer, the
first layer being an active surface cooperative with an immediately
adjacent the first side of the membrane and the second layer being
a supporting structure for the first layer such that an increased
number of electrical current flow paths from the cathode to the
membrane are provided; and
b. the membrane being surface modified on at least the first side
adjacent the cathode so that reduced resistance at the
cathode-membrane junction is achieved to permit cell operation at
current densities greater than about 4.0 kiloamperes per square
meter with a voltage coefficient less than about 0.20 volts per
kiloampere per square meter while maintaining a value for the
constant in a cell voltage equation equal to the linear
extrapolation to zero current density of the slope of the total
cell voltage versus current density plot, wherein the cell voltage
equation is Vcell=Constant+(Voltage Coefficient)(Current
Density).
2. The apparatus according to claim 1 wherein the cathode is a
lower overvoltage cathode.
3. The apparatus according to claim 1 wherein the anode is a lower
overvoltage anode.
4. The apparatus according to claim 2 wherein the cathode is a low
overvoltage cathode with a hydrogen overvoltage of not greater than
about 0.3 volts at about 9.5 kiloamperes per square meter.
5. The apparatus according to claim 3 wherein the anode is a low
overvoltage anode with a chlorine overvoltage of not greater than
about 0.4 volts at about 9.5 kiloamperes per square meter.
6. The apparatus according to claim 1 wherein the voltage
coefficient during operation is from about 0.10 to about 0.20 volts
per kiloampere per square meter.
7. The apparatus according to claim 1 wherein there is no gap
between the first layer of the cathode and the membrane.
8. The apparatus according to claim 1 wherein there is no gap
between the anode and the membrane.
9. The apparatus according to claim 1 wherein there is a gap of
about 1.0 millimeter or less between the first layer of the cathode
and the membrane.
10. The apparatus according to claim 1 wherein the first layer of
the cathode is comprised of a first foraminous metal structure from
about 0.010 to about 0.045 inches thick.
11. The apparatus according to claim 10 wherein the first
foraminous metal structure is selected from the group consisting of
nickel, Raney-nickel or Raney-nickel-molybdenum, lanthanum-nickel
and lanthanum-pentanickel.
12. The apparatus according to claim 10 wherein the first
foraminous metal structure further consists of a coating selected
from the group consisting of Raney-nickel, Raney-nickel-molybdenum,
lanthanum-pentanickel and lanthanum-nickel.
13. The apparatus according to claim 12 wherein the first
foraminous metal structure is a mesh design with a plurality of
openings therein.
14. The apparatus according to claim 1 wherein the first layer of
the cathode is comprised of a reticulate mat of predetermined
thickness.
15. The apparatus according to claim 1 wherein the second layer of
the cathode is comprised of a second foraminous metal structure of
a thickness greater than the first foraminous metal structure.
16. The apparatus according to claim 15 wherein the second
foraminous metal structure is about 0.015 to about 0.045 inches
thick.
17. The apparatus according to claim 16 wherein the second
foraminous metal structure is of an open mesh design with a
plurality of openings therein which are about 0.5 inches by about
1.25 inches.
18. The apparatus according to claim 13 wherein the second
foraminous metal structure is of an open mesh design with a
plurality of openings therein which are larger than the plurality
of openings in the first foraminous metal structure.
19. The apparatus according to claim 1 wherein the second layer of
the cathode is comprised of a separator plate of generally
rectangular shape having a top and a bottom and a first side and a
second side with generally parallel extending support ribs attached
to the first side adjacent the first layer of the cathode between
the top and the bottom.
20. The apparatus according to claim 19 wherein the second layer
further comprises a mesh structure intermediate the support ribs
and the first layer.
21. A method of operating a filter press membrane electrolytic cell
having at least one cathode and one anode sandwiched about a
permselective ion exchange membrane, comprising the steps of
a. operating the cell at greater than a 4.0 kiloampere per square
meter current density at about one atmosphere pressure;
b. maintaining the cell at a voltage coefficient less than or equal
to about 0.20 volts per kiloampere per square meter; and
c. maintaining a value for a constant in a cell voltage equation
equal to the linear extrapolation to zero current density of the
total cell voltage versus current density plot wherein the cell
voltage equation is Vcell=Constant+(Voltage Coefficient)(Current
Density).
22. The method according to claim 21 further comprising maintaining
the anolyte temperature less than or equal to 98.degree. C.
23. The method according to claim 21 further comprising maintaining
no gap between the cathode and the membrane.
24. The method according to claim 23 further comprising maintaining
no gap between the anode and the membrane.
25. The method according to claim 21 further comprising maintaining
a gap of about 1.0 millimeters or less between the cathode and the
membrane.
26. The method according to claim 21 further comprising operating
the filter press membrane cell with a low overvoltage cathode
having a hydrogen overvoltage of not greater than about 0.3 volts
at about 9.5 kiloamperes per square meter.
27. The method according to claim 21 further comprising operating
the filter press membrane cell with a low overvoltage anode having
a chlorine overvoltage of not greater than about 0.4 volts at about
9.5 kiloamperes per square meter.
28. The method according to claim 21 further comprising operating
the filter press membrane cell with a cathode having a first layer
with an active surface and a second layer supporting the first
layer.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to filter press membrane
electrolytic cells. More specifically, it relates to the structure
and operating conditions which permit a filter press membrane cell
to be operated at high current densitites.
Chlorine and caustic, products of the electrolytic process, are
basic chemicals which have become large volume commodities in the
industralized world today. The overwhelming amounts of these
chemicals are produced electrolytically from aqueous solutions of
alkali metal chlorides. Cells which have traditionally produced
these chemicals have come to be known as chloralkali cells. The
chloralkali cells today are generally of two principal types, the
deposited asbestos diaphragm-type electrolytic cell or the flowing
mercury cathode-type.
Comparatively recent technological advances, such as the
development of dimensionally stable anodes and various electrode
coating compositions, have permitted the gap between electrodes to
be substantially decreased. This has dramatically increased the
energy efficiency during the operation of these energy-intensive
units.
The development of a hydraulically impermeable membrane has
promoted the advent of filter press membrane chloralkali cells
which produce a relatively uncontaminated caustic product. This
higher purity product obviates the need for caustic purification
and reduces the need for concentration processing. Initially the
use of a hydraulically impermeable planar membrane has been most
common in bipolar filter press membrane electrolytic cells. Some
filter press membrane cells, especially in the bipolar electrode
design, have attempted to use a dual cathode surface comprising a
first layer of coarse supporting mesh to serve as a current
distributor and a finer mesh cathode screen on top of the coarse
supporting mesh as the second layer. Other cell designs have
recognized the need for obtaining uniform current distribution,
especially in cells of a monopolar design, but have failed to
achieve this for several reasons, for example because of the use in
wide, short cells of a bus bar carrying current across the width of
a cell, but near the cell bottom, so that the electrode material
has to carry the current vertically upwardly in the cell. However,
continual advances have been made in the development of monopolar
filter press membrane cells.
Despite these continued advances in the filter press cell
technology, the high initial capital cost to build an electrolytic
cell facility has deterred large scale construction of these type
of cells in the industry. Attempts to reduce these high capital
costs have recently focused on the ability to operate the cells at
elevated current densities to permit fewer cells to be able to
produce more product than is conventionally produced at lower
current densities in the two to three kiloampere per square meter
range. However, such attempts have met with problems because of the
heat buildup within the operating cell. This heat buildup results
from the resistance that the cell components generate to current
flow through the cell. The cell has metal parts such as conductor
rods, electrode frames, bus bars, the cathodes and the anodes that
contribute to the voltage coefficient resistance, which is the sum
of the resistances of the cell components, the membranes and the
electrolyte to current flow. Filter press membrane cells, in the
past, have had typical hardware or cell component resistances of
approximately 250 millivolts at current densities in the 3
kiloampere per square meter range.
As the heat builds within the cell, the electrolyte temperature
increases and can even reach the boiling point. This elevated
temperature can cause the water to be removed from the cell, such
as by evaporation or boiling off, especially in the anolyte, faster
than it is replaced. The permselective ion exchange membranes are
also affected by this elevated temperature. The polymer chains on
current membranes can delaminate from each other because of
elevated operating temperatures, which will cause blisters in the
membrane. The membranes also can rupture or burst due to the water
boiling within the membrane because of the heat generated by the
electrical resistance within the membrane. In order for the
membrane to function properly, the water must remain in the liquid
phase. The elevated temperature and the boiling of the water can
cause the membranes to delaminate when a cell is operated at a
current density above 4.0 kiloamperes per square meter over a
period as short as a few minutes, depending upon cell size.
These problems are solved in the design of the present invention by
controlling the voltage coefficient or summation of resistances of
the cell, expressed in terms of the current density, below a
predetermined level to obtain a heat and material balance which,
because of the lower cell resistance, permits the cell to be
operated at higher current densities.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a filter press
membrane electrolytic cell that may be operated at current
densities greater than about 4.0 kiloamperes per square meter.
It is another object of the present invention to provide a filter
press membrane electrolytic cell that employs a cathode that
increases the electrical current flow paths between the cathode
surface and the membrane.
It is yet another object of the present invention to provide a
filter press membrane cell that achieves substantially uniform
current distribution and substantially constant vertical
electrolyte concentration within each electrode.
It is a feature of the present invention that a dual cathode having
a first layer with an active surface and a second layer with a
supporting structure is employed.
It is another feature of the present invention that low overvoltage
cathodes and surface modified membranes are employed to control the
heat balance within the cell.
It is still another feature of the present invention that the cell
operating temperature is maintained at or below 98.degree. C. at
atmospheric pressure and the total voltage coefficient of the cell
is less than about 0.20 volts per kiloampere per square meter.
It is an advantage of the present invention that a heat and
material balance is obtained to permit the filter press membrane
electrolytic cell to be operated at high current densities.
It is another advantage of the present invention that more product
can be produced with a fewer number of cells.
It is another advantage of the present invention utilizing the
alternative embodiment of a filter press membrane electrolytic cell
with a monopolar plate type of electrode design that the voltage
drop and current distribution efficiencies obtained are as close to
those obtained in a biopolar filter press membrane cell design as
appears to be practically possible.
These and other objects, features and advantages are obtained in a
filter press membrane electrolytic cell having at least one cathode
and one anode sandwiched about a permselective ion exchange
membrane with a modified or treated surface adjacent at least the
cathode which, in conjunction with a dual cathode having a first
layer with an active surface and a second layer with a supporting
structure, permits the cell to be operated at current densities
greater than 4.0 kiloamperes per square meter with a voltage
coefficient that is less than about 0.20 volts per kiloampere per
square meter while maintaining a value for the constant in the cell
voltage equation, Vcell=Constant+(Voltage Coefficient) (Current
Density), equal to the linear extrapolation to zero current density
of the slope of the plot of the total cell voltage versus the
current density curve.
BRIEF DESCRIPTION OF THE DRAWINGS
The advantages of this invention will become apparent upon
consideration of the following detailed disclosure of the
invention, especially when it is taken in conjunction with the
accompanied drawings wherein:
FIG. 1 is a graphic plotting of the voltage versus the current
density showing the total cell voltage plot, the slope of which
equals the voltage coefficient of the cell above a 3.0 kiloampere
per square meter current density;
FIG. 2 is a graphic plotting of a second case of the voltage versus
the current density showing the total cell voltage plot, the slope
of which equals the voltage coefficient of the cell above a 3.0
kiloampere per square meter current density;
FIG. 3 is a side elevational view of an intermediate cathode with
the dual cathode's first and second layers removed;
FIG. 4 is an enlarged, partial sectional view taken along the lines
4--4 of FIG. 3 with the dual cathode layers shown and a conductor
rod partially shown;
FIG. 5 is a diagrammatic illustration of a plate type of cathode
that may be employed as an alternative embodiment;
FIG. 6 is a graphic plotting of the cell voltage versus the current
density showing the voltage coefficiency for a cell of the
alternative embodiment that is operated at a current density of up
to about 10 kiloamperes per square meter;
FIG. 7 is a graphic plotting of the moles of water lost through
evaporation in a cell versus the temperature of the cell chlorine
gas/anolyte flow streams; and
FIG. 8 is a graphic plotting of the chlorine gas temperature versus
the voltage coefficient for a monopolar filter press membrane cell
operated at high current densities.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 3 shows the structure of a cathode 10 minus the electrode
surfaces which may be employed in a cell of the design
incorporating the instant invention to achieve operating conditions
with current densities in excess of 4.0 kiloamperes per square
meter employing surface treated or modified ion selective membranes
in a filter press cell type of configuration. The cathode 10 has a
frame that is comprised of components 11, 12, 14 and 15. Frame
components 12 and 15 extend generally vertically and are parallelly
spaced apart during operation of the cell. Frame components 11 and
14 are positioned generally horizontally during the cell
operation.
The top frame component 11 is seen as having a sample port 18 and a
cathode riser 16 projecting from the top thereof. An anode (not
shown) may have a corresponding riser and sample port to permit
fluid flow between the appropriate gas-liquid disengager (not
shown) and the corresponding electrode. The risers are generally
utilized to carry the appropriate electrolyte fluid with the
accompanying gas, either anolyte with chlorine gas or catholyte
with hydrogen gas, to the appropriate disengager (not shown)
mounted on top of a filter press membrane cell. External
circulation is employed to circulate electrolyte from the
appropriate disengager through infeed manifolds (not shown) back
into the electrodes through infeed pipes.
The bottom frame component 14 is shown having a catholyte infeed
pipe 19 that extends upwardly through the bottom into the interior
of the cathode formed between the opposing electrode surfaces. The
catholyte infeed pipe 19, as well as the corresponding anolyte
infeed pipe (not shown), are connected to infeed manifolds (also
not shown) to permit the anolyte and catholyte fluids to be fed
upwardly through the bottom of the appropriate electrode
frames.
A series of lifting lugs 20 are spaced about the exterior of the
frame components 11, 12, 14 and 15. These lifting lugs 20 permit
the cathode 10 to be easily lifted into position for assembly. A
similar structure can be found on the anode frames (not shown).
Similarly, a series of spacer blocks 21 are positioned about the
exterior of the frame components 11, 12, 14 and 15. These spacer
blocks 21 are positioned so that they are opposite and adjacent
corresponding spacer blocks on the adjacent anode (not shown) so
that spacers may be placed between the pairs of spacer blocks to
assure the proper interelectrode gap is obtained uniformly about
the assembled cell in a manner that is well known in the art.
The cathode 10 is seen as having conductor rods 22 extending
generally horizontally through one of the generally vertically
extending frame components, in this case frame component 12.
Appropriately fastened, such as by welding, to each of the
conductor rods 22 are a plurality of vertically extending current
distributor ribs 24 which are spaced generally equally across the
width of the cathode to permit uniform distribution of the current.
The conductor rods 22, similarly, are generally equally distributed
across the vertical height of the cathode 10 to permit the current
to be introduced generally uniformly across the full height of the
cathode 10.
As seen in FIG. 4, each of the frame components, such as frame
component 15 is generally U-shaped with a covering plate 25
covering the top of the U. The dual cathode, indicated generally by
the numeral 26, is seen as comprising a first layer 28 and a second
layer 29 on both sides of the cathode 10. The first layer 28 is the
primary active surface and is a foraminous metal structure,
preferably a mesh formed of expanded metal. The second layer 29 is
a foraminous metal supporting layer, also preferably a mesh formed
of expanded metal, with larger openings than in the first layer 28
to promote the passage of the electrolytically generated gas
bubbles therethrough. The openings in the second layer 29 optimally
are four times the size of the openings in the first layer 28 with
the primary active surface.
Second layer 29 is preferably fastened to the current distributor
ribs 24, such as by welding. The current distributor ribs 24 (only
one of which is shown in FIG. 4) are fastened, as described above,
to the conductor rods 22 (only one of which is partially shown).
The second layer 29 is seen as being curved inwardly toward the
center of the cathode 10 interiorly of the inner wall or base 30 of
the U-shaped frame component 15.
The first layer 28 of the cathode is shown as extending over the
space between this inwardly curved portion 31 of the second layer
29 and the base 30 of the frame component 15. Thus, the second
layer 29 does not contact any of the frame components 11, 12, 14 or
15. The first layer 28 may be fastened, such as by spot welding, to
the leg portions 32 of the U-shaped frame components. The membrane,
not shown in FIG. 4, is then placed adjacent the first layer 28 on
both sides of the cathodes between the adjacent anodes to form a
cathode-membrane-anode sandwich.
The anodes employed in a cell of the design incorporating the
present invention (not shown) may be similar to the cathode 10
design, employing either a dual layer active surface or a single
layer active surface.
It should be noted that both electrodes, the cathode 10 and the
anode (not shown), are of the low overvoltage type. That is, in an
effort to reduce the working voltage of an electrolytic cell and,
specifically, the overvoltage at both the anode and the cathode,
low overvoltage cathodes and anodes are employed for the active
surfaces. The cathode or the anode may comprise a solid or
perforated plate, a rod, a foraminous structure or a mesh of any
shape. While the preferred cathode structure has been described as
being a mesh, it could equally well be a reticulate mat as long as
a supporting structure of some type is employed. Such a reticulate
mat can be made from a cathode substrate comprised of a conductive
metal, such as copper or nickel, plated with an intermediate
coating of a porous dendritic metal and an outer coating of a low
overvoltage material, such as Raney nickel or other appropriate
alloy. The anode may be formed from a suitable valve metal, such as
titanium or tantalum, which has a suitable coating with low
overvoltage characteristics, such as ruthenium oxide, platinum or
other coatings from the platinum group metals, a platinum group
metal oxide, an alloy of a platinum group metal, or a mixture
thereof. The term platinum group metal as used herein means an
element from the group consisting of ruthenium, rhodium, palladium,
osmium, iridium and platinum.
An alternative embodiment of the cathode structure is shown in FIG.
5 wherein a cathode, indicated generally by the numeral 34 is seen
comprising a copper plate 35, a separator plate 36 with vertically
extending hollow risers 38 and generally rectangularly shaped frame
components 39. A mesh or first layer 40 is placed atop the
supporting layer formed by the separator plate 36 with its riser
38. Alternately, a supporting mesh second layer (not shown) can be
placed over the risers 38 between the risers 38 and the first layer
40. A surface treated or surface modified mambrane 41, only one of
which is shown, is then placed against each of the active surface
layers 40. A rib type of structure, instead of hollow riser, could
equally well be employed similar in structure to the ribs 24 of
FIG. 3.
In the alternative embodiment, the cathode mesh is preferably 0.025
inches thick and formed of a Raney nickel-molybdenum alloy, nickel
or codeposited Raney nickel on nickel with three millimeter by six
millimeter openings. The thickness could be as low as 0.01 inches
thick. The mesh support structure should be thicker, formed from a
nickel construction with a thickness of about 0.035 to about 0.045
inches with about 0.5 inch by about 1.25 inch openings. It is
feasible, however, to use a mesh support structure that is as thin
as about 0.15 inches and still retain sufficient mechanical
elasticity properties that are required with the compression forces
applied during cell assembly. The first layer 40 in this design is
welded to the risers 38 or to other suitable supporting structure,
such as the mesh support structure. Where a cathode mesh of thinner
proportion is employed the first layer 40 is maintained in contact
with the reiser 38 or other suitable supporting structure by
pressure and no welding is employed.
The anode (not shown) preferably is of similar structure but would
employ titanium in the separator plate in combination with a
titanium mesh first layer with the same thicknesses and openings or
slightly thicker with larger mesh openings and the mesh layer is
welded to the risers.
An appropriate surface modified or surface treated memberane may be
selected from those available under the Nafion trademark or the
Flemion trademark employing a tin oxide, titanium oxide, tantalum
oxide, silicon oxide, zirconium oxide or an iron oxide, such as
Fe.sub.2 O.sub.3 or Fe.sub.2 O.sub.4, coating on the anode and the
cathode sides. Alloys of these elements, as well as hydroxides,
nitrides or carbide powders could also be employed. Additional
elements suitable for forming a porous layer on the cathode side
are silver, stainless steel and carbon. This surface treatment
provides a and liquid permeable porous non-electrode layer that
reduces the buildup of gas bubbles, such as hydrogen the cathode
side and chlorine on the anode side, by changing the nature of the
membrane's treated surface from hydrophobic to hydrophilic to
promote the gas release properties of the membrane.
The membrane can be positioned from the adjacent electrode active
surfaces by either a finite gap or by no gap, commonly known as
zero gap. However the greater the gap or distance between the
membrane and the electrode surface, such as the cathode, the
greater is the voltage drop between the electrode surface and the
membrane because the current must pass through more of the
separating electrolyte. As current densities increase this voltage
drop correspondingly increases. For example, with a two millimeter
gap between the cathode and a surface modified membrane, such as a
Flemion.RTM. 755 or 757 or 775 membrane, at 3.0 kiloamperes per
square meter current density a 0.065 volt drop was recorded. At 4.5
kiloamperes per square meter the drop was 0.095 volts; at 6.0
kiloamperes per square meter the drop was 0.216 volts. Thus, at
higher current densities the voltage drop increases across a gap.
At corresponding amperage values in an equivalent cell operated
with a zero gap, the voltage drop between this cathode and the
membrane was zero or negligible, at least being below the
recordable tolerances of the measuring apparatus.
The current that flows through a filter press membrane electrolytic
cell causes a voltage as it passes through each component of the
cell. The total cell voltage, then, is the sum of the minimum
voltage to initiate the electrolytic reaction, the voltage at the
membrane/electrolyte surface junctions, the anode overvoltage, the
voltage of the anolyte, the voltages of the membrane, the voltage
of the catholyte, the cathode overvoltage and the voltage of the
cell hardware. The voltage at the membrane/electrolyte surface
junctions and the minimum voltage to initiate the reaction are
independent of the current density and may be expressed as
constants. The other voltage components increase with increasing
current density, thereby increasing the heat generated within the
cell due to the increased product of current and resistance.
To be able to operate at high current densities the increase of
voltage and current density must be maintained in a linear
relationship with a voltage coefficient that is less than or equal
to about 0.20 volts per kiloampere per square meter so that the
increase in voltage is controlled with respect to the current
density. This relationship may be expressed as an equation,
Vcell=Constant+(Voltage Coefficient) (Current Density).
The constant in the equation is equal to the sum of the minimum
voltage to initiate the reaction and the membrane/electrolyte
surface junction voltage. This constant is the intercept on the
voltage axis of the cell voltage versus current density plot and is
graphically obtained by extrapolating back to zero current density
the linear plot of the cell voltage versus the current density. The
voltage coefficient previously has been described as representing
the sum of the resistances of the cell components, the membranes
and the electrolyte to current flow. Graphically, the voltage
coefficient is equal to the slope of the plot of the total cell
voltage versus the current density.
In commercial electrolyzers, however, the constant in the cell
voltage equation is not taken to be equal to the theoretical
decomposition potential of the following reactions:
This calculated decomposition potential will vary depending upon
the temperature at which the reactions occur, and the
concentrations and pressures of the reactants and products. Such
calculated values can range from 2.15 to 2.30 volts.
The decomposition potential can be calculated from the equation
where .DELTA.G.sub.o is defined as the total change in chemical
potentials of the reactants and products involved in the
accompanying reaction in the cell, n is the number of electrons
transferred per mole of ions involved, F is the Faraday constant
and E.sub.o is the electrical potential in the reaction.
At current densities below 1.0 KA/m.sup.2, the relationship between
the total cell voltage and the current density becomes non-linear.
The total cell voltage drops non-linearly to the decomposition
potential at zero current density. This decomposition potential at
the cell operating conditions for FIGS. 1, 2 and 6 is about 2.23
volts. The non-linear portion of the total cell voltage versus
current density curves is shown by the dotted lines in FIGS. 1, 2
and 6. While not shown graphically, the anode and cathode voltages
are similarly non-linear at current densities below 1.0
KA/m.sup.2.
The following examples will illustrate how an electrolytic cell
employing a permselective membrane can operate at high current
densities, such as up to 10 kiloamperes per square meter, if the
voltage coefficient is kept below about 0.20 volts per kiloampere
per square meter.
As previously mentioned heat is generated as current (I) flows
through the resistance (R) in the cell and can be measured as a
voltage. Heat generation (IR) will increase with an increase in
either resistance or current density. This heat must be compatible
with the overall energy and material balance in the operating cell.
This IR heat can increase the temperature of the anolyte and
catholyte fluids or can boil off water from the anolyte and
catholyte fluids if the temperature increase is sufficient.
As the voltage coefficient in an operating cell increases above
about 0.20 volts per kiloampere per square meter at a high current
density, for example 10 kiloamperes per square meter, the two most
important energy and material balance factors controlling cell
operation appear to be the increase in temperature for the chlorine
gas/anolyte flow streams and the increase in steam content in or
with the chlorine gas.
Operation of a cell at higher current densities is generally
obtained by a gradual buildup of the current density. This
typically is obtained through the use of a cell jumper switch that
allows stepwise increases in the current density. For example, the
current density can be increased at 1/2 kiloampere per square meter
increments every thirty seconds until the desired current density
is obtained.
EXAMPLES
The following examples are presented without any intent to limit
the scope of the invention while illustrating the operation of a
filter press membrane cell at a high current density greater than
about 4.0 kiloamperes per square meter of electrode surface per
electrode with a voltage coefficient of less than about 0.20 volts
per kiloampere per square meter of electrode surface per
electrode.
EXAMPLE 1
A monopolar filter press membrane cell for the production of
chlorine and caustic was operated with one cathode and one anode,
both of the low overvoltage type. The cathode employed the dual
layer design with the first layer or primary active surface being
Raney-nickel-12% molybdenum and the second or supporting layer
being nickel-200 mesh. The anode was a pH stabilized Conradty
anode. A Nafion.RTM. brand DuPont membrane with a modified or
treated surface was positioned between the cathode and anode
surface with no electrolyte gap therebetween. Each electrode and
the membrane had 500 square centimeters of active surface area.
The cell was operated with approximately 200 grams per liter of
anolyte concentration at 90.degree. C. to produce caustic with a
concentration of about 32.5%. The current to the cell was
incremented gradually from startup until operation at a current
density of 9.5 kiloamperes per square meter was obtained. Average
voltage readings are shown in the following table with a standard
deviation to reflect voltage fluctuations that occurred during
operation. The cell was operated at one atmosphere.
A shutdown of the cell occurred after 47 days of operation, after
which the cell was restarted and operated for an additional 14
days. However, some unknown abnormal event occurred during the
shutdown and/or startup procedure which adversely affected the cell
voltage. For the 23 days of operation of the cell at 9.5
kiloamperes per square meter prior to the shutdown, the average
anode voltage was about 0.34 volts and the average cathode voltage
was about 0.22 volts.
______________________________________ CURRENT CURRENT DAYS DENSITY
VOLTAGE EFFICIENCY OF (KA/m.sup.2) (VOLTS) % OPERATION
______________________________________ 3 2.89 .+-. 0.01 96.53 3 6
3.31 .+-. 0.03 93.66 .+-. 0.92 21 9.5 3.83 .+-. 0.04 89.01 .+-.
1.13 23 9.5 3.89 .+-. 0.07 88.48 .+-. 0.88 47
______________________________________
The hydrogen overvoltage at the low overvoltage cathode for 23 days
of operation at 9.5 KA/m.sup.2 during the first 46 days of
operation prior to the cell shutdown was measured as an average of
about 0.22 volts. The chlorine overvoltage at the low overvoltage
anode for the same period was measured as an average of about 0.34
volts. Operation of the cell at 9.5 KA/m.sup.2 did not have the
hydrogen overvoltage at the cathode exceed about 0.30 volts nor the
chlorine overvoltage at the anode exceed about 0.40 volts. The
second set of values at 9.5 KA/m.sup.2 represent the average of the
values obtained for the total of the 47 days the cell was operated
at 9.5 KA/m.sup.2, including 14 days of operation after the
temporary cell shutdown.
The graphic plotting in FIG. 1 is the result of the plotting of the
individual daily data used to compile the above summary table. The
plot labelled A is the total cell voltage versus the current
density, while plot B represents the anode and cathode voltage
contribution combined and plot C represents just the cathode plot.
Both plots B and C include the minimum reaction voltage and the
membrane/electrolyte junction voltage. The slope of plot A then
represents the voltage coefficient for the cell which calculates to
0.145 volts per kiloampere per square meter and is based upon a
linear extrapolation to zero current density from current densities
above 1.0 KA/m.sup.2.
EXAMPLE 2
A monopolar filter press membrane cell for the production of
chlorine and caustic was operated with one cathode and one anode,
both of the low overvoltage type. The cathode employed the dual
layer design with the first layer or primary active surface being
lanthanum-containing layer on nickel and the second or supporting
layer being nickel-200 mesh. The anode was a DSA.RTM. Eltech
Corporation anode. A Flemion.RTM. brand Asahi Glass membrane with a
modified or treated surface was positioned between the cathode and
anode surface with no electrolyte gap therebetween. Each electrode
and the membrane had 500 square centimeters of active surface
area.
The cell was operated with approximately 200 grams per liter of
anolyte concentration at 90.degree. C. to produce caustic with a
concentration of about 35.5%. The current to the cell was
incremented gradually from startup until operation at a current
density of 9.5 kiloamperes per square meter was obtained. Average
voltage readings are shown in the following table with a standard
deviation to reflect voltage fluctuations that occurred. The cell
was operated at one atmosphere.
______________________________________ CURRENT CURRENT DAYS DENSITY
VOLTAGE EFFICIENCY OF (KA/m.sup.2) (VOLTS) % OPERATION
______________________________________ 3 2.96 .+-. .04 95.37 .+-.
.96 23 6 3.40 .+-. .04 93.92 .+-. .78 21 9.5 3.98 .+-. .02 92.76
.+-. .96 17 ______________________________________
The hydrogen overvoltage at the low overvoltage cathode for the
total days of operation was measured as an average of about 0.30
volts and the chlorine overvoltage at the low overvoltage anode for
the same period was measured as an average of about 0.38 volts.
Operation of the cell at 9.5 KA/m.sup.2 did not have the hydrogen
overvoltage at the cathode exceed about 0.31 volts nor the chlorine
overvoltage at the anode exceed about 0.40 volts.
The graphic plotting in FIG. 2 is the result of the plotting of the
individual daily data used to compile the above summary table. The
plot labelled A is the total cell voltage versus the current
density, while plot B represents the anode and cathode voltage
contribution combined and plot C represents just the cathode plot.
Both plots B and C include the minimum reaction voltage and the
membrane/electrolyte junction voltage. The slope of plot A then
represents the voltage coefficient for the cell which calculates to
0.157 volts per kiloampere per square meter and is based upon a
linear extrapolation to zero current density from current densities
above 1.0 KA/m.sup.2.
EXAMPLE 3
A filter press membrane cell of the alternative embodiment with one
plate cathode and one plate anode was operated with a Nafion.RTM.
brand DuPont membrane. The anode was DSA.RTM. anode from Eltech
Corporation with 1.5 square meters of surface area. The dual
cathode had an active surface of Raney-nickel-12% molybdenum in the
first layer or primary active surface and a second or supporting
layer of nickel-200 mesh. The membrane and cathode both had 1.5
meters of active surface area. There was no electrolyte gap between
the anode, membrane and cathode.
The cell was operated with approximately 230 grams per liter of
anolyte concentration at current densities of about 4.0, 7.1 and
9.9 kiloamperes per square meter (KA/m.sup.2). At 4.0 KA/m.sup.2
the operating temperature for 2 days averaged about 77.degree. C.
At about 7.1 KA/m.sup.2 the operating temperature for 20 days
averaged about 90.degree. C. with an average caustic concentration
of about 32.35%. At about 9.9 KA/m.sup.2 the operating temperature
for 9 days averaged about 92.degree. C. with an average caustic
concentration of about 32.52%.
______________________________________ CURRENT CURRENT DAYS DENSITY
VOLTAGE EFFICIENCY OF (KA/m.sup.2) (VOLTS) % OPERATION
______________________________________ 4.0 3.33 .+-. .02 -- 2
7.1/7.2 3.91 .+-. .02 90.05 .+-. .67 17 9.9 4.40 .+-. .01 90.03
.+-. .96 3 ______________________________________
The graphic plotting in FIG. 6 uses the Constant in the cell
voltage equation equal to the linear extrapolation to zero current
density of the slope of the total cell voltage plot from current
densities above 1.0 KA/m.sup.2 and reveals the averages shown above
for the data readings taken over the number of days indicated.
Multiple readings were taken on each day with the exception of the
first day of operation. The voltage coefficient was 0.181 volts per
kiloampere per square meter and illustrates that by maintaining the
voltage coefficient at this level a monopolar filter press cell can
operate at high current densities.
The current efficiency was not calculated for the first two days of
operation. The cell was operated for 19 additional days at a
current density that fluctuated between 9.9 and 10.0 KA/m.sup.2
after the 3 days for which data were averaged at 9.9 KA/m.sup.2 to
form the plot shown in FIG. 6. The data for these days also
conforms to the plot shown in FIG. 6 so that the voltage
coefficient is substantially unchanged. FIGS. 7 and 8 illustrate
the effect of an increase in cell operating temperature on the
moles of water lost due to evaporation from the chlorine
gas/anolyte flow streams and the effect on the cell operating
temperature by the increase in cell voltage coefficient from 0.12
to 0.34 volts per kiloampere per square meter in a filter press
membrane cell. These figures illustrate the importance of keeping
the voltage coefficient below about 0.20 volts per kiloampere per
square meter to maintain the cell operating temperature below about
98.degree. C. and preferably below about 95.degree. C. These graphs
are based upon a cell with 15 square meters of membrane and anode
and cathode surface area each operated at 95% current efficiency
with a 10.0 kiloampere per square meter current density and a total
cell load of 150 kiloamperes. The brine feed rate was 10.75 gallons
per minute with an inlet brine temperature of 30.degree. C.
Reviewing the data and graphical plots of Examples 1-3 will reveal
the relationship between the theoretical decomposition potential
and actual potential in electrolyzers where the voltage coefficient
is kept below about 0.20 volts per kiloampere per square meter
(KA/m.sub.2).
The total cell potential versus current density plot or curve A in
Example 1, shows that the cell voltage points lie on a straight
line which, when extrapolated to zero current density, has an
intercept on the cell voltage axis of 2.451 volts. The linear
equation for the cell voltage of this straight line plot is
2.451+0.145 i, where i is the current density expressed in
KA/m.sup.2. The slope or voltage coefficient is 0.145
volts/KA/m.sup.2.
The total cell potential versus current density plot or curve A in
Example 2, shows that the cell voltage points lie on a straight
line which, when extrapolated to zero current density, has an
intercept on the cell voltage axis of 2.477 volts. The linear
equation for the cell voltage of this straight line plot is
2.477+0.157 i, where i is the current density expressed in
KA/m.sup.2. The slope or voltage coefficient is 0.157
volts/KA/m.sup.2.
The total cell potential versus current density plot or curve in
Example 3, shows that the cell voltage points lie on a straight
line which, when extrapolated to zero current density, has an
intercept on the cell voltage axis of 2.607 volts. The linear
equation for the cell voltage of this straight line plot is
2.607+0.181 i, where i is the current density expressed in
KA/m.sup.2. The slope or voltage coefficient is 0.181
volts/KA/m.sup.2.
The intercept value on the voltage axis of Examples 1 and 2 are
different, reflecting the difference in the cathode activity, and
are not equal to the aforementioned decomposition potential of in
the range 2.15-2.3 volts. The slopes of the curves are different
than if they were plotted between cell voltage intercepts based on
the theoretical decomposition potential and actual voltage data
points above 1.0 KA/m.sup.2, reflecting the differences in the
voltages caused by electrolyte concentration gradients across the
membranes and electrode over-potentials.
From the Examples it is clear that the cell voltage is best
calculated from a voltage coefficient by specifying the value of
the constant of the equation since that constant can be different
for varying combinations of cell structure, membrane, and cathode.
Linearly extrapolating to zero current density the total cell
voltage versus current density plot from current densities above
1.0 KA/m.sup.2 provides the best method for specifying the constant
in the cell voltage equation for commercial electrolyzers.
As seen in FIG. 7, at 98.degree. C. the number of moles of water
evaporating increases dramatically as more of the IR heat energy in
the chlorine gas/anolyte flow streams evaporates water instead of
further increasing the temperature of the flow streams. Such rapid
evaporation also creates serious problems with disengaging gases
from the anolyte. FIG. 8 shows that the voltage coefficient above
0.20 volts per kiloampere per square meter corresponds to a cell
operating temperature that exceeds 98.degree. C.
Part of the reason for the lower voltage coefficient is the reduced
hardware loss obtained by the cell hardware used in the instant
invention. For example, in the alternative embodiment of FIG. 5
where a plate type of electrode is used the voltage drop for the
total cell hardware in a cell with 10.0 kiloampere per square meter
current density and a total cell load of 15 kiloamperes is
calculated to be 92.3 millivolts.
This design calculation is broken out as follows:
______________________________________ Voltage Drop Anode (MV)
______________________________________ Plate 18.2 Mesh Support 11.4
Active Mesh 31.8 Total 61.4 ______________________________________
Voltage Drop Cathode (MV) ______________________________________
Plate 18.0 Mesh Supports 2.3 Active Mesh 10.6 Total 30.9
______________________________________
The lower the voltage drop, the less resistance encountered by the
current as it flows through the cell and, therefore, the lower is
the voltage coefficient. Thus, the heat generated will also be less
and will contribute less to the needed heat balance to operate the
cell at high current densities.
While the preferred structure in which the principles of the
present invention have been incorporated is shown and described
above, it is to be understood that the invention is not to be
limited to the particular details thus presented, but in fact,
widely different means may be employed in the practice of the
broader aspects of the method of this invention. For example, the
cathode can employ a primary active surface or first layer being
lanthanum-pentanickel-nickel or utilize coatings on a foraminous
metal structure of the first layer metals of Raney-nickel,
Raney-nickel-molybdenum, lanthanum-pentanickel, lanthanum nickel or
alloys thereof. The scope of the appended claims is intended to
encompass all obvious changes in the details, materials and method
of utilizing the parts which will occur to one of skill in the art
upon a reading of the disclosure.
* * * * *